1. Field of the Invention
The invention relates to a photopolymerizable mixture for the production of color proofs containing a) a polymeric binder with salt-forming groups or a binder mixture with at least one polymer with salt-forming groups, b) at least one photopolymerizable monomer and c) a photoinitiator or a photoinitiator system; a photopolymerizable recording material; and a process for the production of color proofs.
2. Description of the Related Art
Reproduction technology makes use of screen color separations as the masters in the production of offset or letterpress printing plates. Prior to the exposure of the printing plates, color testing methods are employed to examine the color separations in order to ensure that the prints obtained from the printing plates that have been exposed through said color separations accurately reproduce the master in terms of tone value.
Such color testing methods employ light-sensitive recording materials in which differences in the tackiness of the exposed and unexposed areas of the light-sensitive layer are used to generate the image. Thus, for example, U.S. Pat. Nos. 3,060,024; 3,620,726; 3,582,327; 3,649,268; 4,734,356; 4,849,322; 4,892,802 and 4,948,704 disclose a reproduction method which makes use of a tacky photopolymerizable recording material consisting of a substrate, of a photopolymerizable layer containing at least one addition-polymerizable monomer and a photopolymerization initiator as well as a cover film. This cover film is pulled off, the material is laminated onto an image carrier and hardened by means of imagewise exposure, as a result of which the exposed image areas lose their tackiness. The substrate can be removed either before or after exposure, depending on the material.
The latent image is then made visible by applying appropriate toner materials which only adhere to the unexposed, tacky areas, and can be removed from the non-tacky image areas after application. This method yields positive, optionally color images of the master, whose appearance is similar to images obtained with printing inks.
The toner materials, which consist primarily of fine-particle powders, can be applied by dusting the toner onto the surface that has been exposed imagewise. According to another embodiment, the toner can also be loosely bonded onto a separate carrier and transferred by placing this carrier in contact with the surface that has been exposed imagewise. U.S. Pat. Nos. 4,806,451; 5,126,226; 5,210,001; 5,292,622; 5,372,910 as well as U.S. Pat. No. 4,935,331 describe color testing methods which employ transfer layers specially designed for this purpose.
Even though these prior art methods can fulfill the most important requirements of the printing industry, such as high resolution, good reproduction of the tone value and low dot gain, the known materials, especially those which contain the monomers of the (meth)acrylated bisphenol-A-epoxy-resin type described as the especially preferred material in U.S. Pat. Nos. 4,734,356; 4,849,322; 4,892,802 and 4,948,704, adhere excessively to the carrier film, which leads to holes in the photopolymeric layer, so-called pinholes, on the paper, or else to "reverse adhesion", that is to say, the photopolymeric film peels off the carrier film in differently sized pieces when the cover film is pulled off so that it is not possible to manufacture useable color proofs.
Photopolymerizable layers which do not contain monomers of the (meth)acrylated bisphenol-A-epoxy-resin type do not show these defects. However, the oxygen sensitivity, especially of thin photopolymerizable layers, increases so much that the layer of air trapped between the paper and the photopolymerizable layer prevents photopolymerization to such an extent that the surface of the layer remains tacky and picks up toner even in the exposed areas. This defect is known as "pepper stain".